Fig. 8.21 ). A tuning curve is a plot of the threshold for activation of the nerve fiber by different sound frequencies. The major factor that influences the activity of individual afferent fibers is the location along the basilar membrane of the hair cells that they innervate. The location of those hair cells is important because for any given sound frequency, there is Distance from stapes (mm) a site of maximum displacement of the basilar membrane B as the pressure wave travels along its length (see
Fig. 8.19 ). Typically, tuning curves are sharp near the characteristic
CHAPTER 8 The Special Senses
Base20,0007,0005,0001,0004,0003,0006004002002,0001,500800Apex •Fig. 8.20 Thetonotopicmapofthecochlea.(RedrawnfromStuhlmanO.An Introduction to Biophysics. NewYork:JohnWiley&Sons;1943.) frequency, but they broaden at high sound pressure levels. Tuning curves can have excitatory and inhibitory areas (see
Fig. 8.21A ). The sharpness of the excitatory regions may reflect inhibitory processes.
The different features of an acoustic stimulus are encoded in the discharges of cochlear nerve fibers. Duration is signaled by the duration of activity; intensity is signaled both by the amount of neural activity and by the number of fibers that discharge. For low-frequency sounds, the frequency is signaled by the tendency of an afferent fiber to discharge in phase with the stimulus (phase locking; see
Fig. 8.22A ). If the tone is much more than 1 kHz, a single fiber cannot discharge with every cycle, but phase locking can also occur for sounds with periods shorter than the absolute refractory period of the afferent fiber. This allows the CNS to detect higher frequency information from the activity of a population of afferent fibers, each of which discharges in phase with the stimulus and which, as a group, signal the frequency of the stimulus (see
Fig. 8.22B ). This observation is the basis of the frequency theory of hearing.
For still higher frequencies (>5000 Hz), the place theory dominates: the CNS interprets sounds that activate afferent fibers supplying hair cells near the base of the cochlea as being of high frequency. Thus both the place and the frequency theories are necessary to explain the frequency coding of sound (duplex theory) across the entire range from 20 to 20,000 Hz.
2 A 7 10 154 –80 –60 –40 –20 0 Frequency (kHz) E I I Threshold intensity (dB) B 0.2 1 2052 100.5 Frequency (kHz) –100 –80 –60 –40 –20 0 Threshold intensity (dB) •Fig. 8.21 Tuning Curves of Neurons in the Auditory System. Tuningcurvescanbeconsideredasreceptivefieldplots.A, Tuningcurvewithcentralexcitatoryfrequencies(E)andflankinginhibitoryfrequencies(I).B, Tuningcurvesforcochlearnervefibers.(A, RedrawnfromArthurRM,etal.J Physiol [Lond] 1971;212:593.B, RedrawnfromKatsuiY.In:RosenblithWA[ed].Sensory Communication. Cambridge,MA:MITPress;1961.)